1. Field of the Invention
The invention relates generally to flying spot optical scanners, and in particular to scanners capable of reading badly printed or dot matrix symbols.
2. Background Art
In known flying spot optical scanners using a linear or raster scanning pattern a field is scanned by traversing a scanner beam over a set of horizontal lines effectively covering the field. Problems often arise when the scanner is reading indicia such as bar code symbols where ink spread and voids have been introduced during the printing process, for example when the indicia includes dot matrix printed symbols, as well as dirty or damaged symbols. Such ink spread and voids may lead to errors or inaccuracies in reading the indicia.
Attempts have been made to solve this problem by varying the optical properties of the spot such that the scanner produces an elliptical spot having its longer axis in the vertical direction. Such an arrangement provides improved reading of indicia such as bar code symbols having vertical bars and spaces relative to the scanning direction.
Another problem associated with known optical scanners is the presence of speckle noise caused by laser speckle, an inherent property of a semiconductor laser diode spot beam, or by the interference of the laser scanning beam arriving at the target with the laser scanning beam reflected from the target, the interference taking place effectively at the surface of the target. Such a problem arises in particular with small detectors such as might be utilized in a two dimensional sensor array or CCD scanner into which the pixel size of a detector area is very small. In such an embodiment, a speckle or bright spot overlapping the pixel can generate an erroneous reading for that pixel.